It is known that coating prosthetic implant devices such as porous coated orthopaedic prostheses, artificial teeth and the like with an oxide or phosphate coating improves the effectiveness and biocompatibility of the devices, by stimulating bone ingrowth or even bonding chemically to the bone structure. Oxide coatings include alumina and zirconia and phosphate coatings include calcium phosphate (such as .alpha. or .beta. tricalcium phosphate Ca.sub.3 PO.sub.4 or Ca.sub.5 (PO.sub.4).sub.3-x (CO.sub.3).sub.x (OH).sub.1+x where x is 0.2 or less) and more particularly calcium hydroxyapatite Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2. These coatings are generally of the order of 60 .mu.m thick and it has even been suggested that calcium hydroxyapatite (CHA) coatings on hip implants significantly reduces "mid-thigh pain" in the immediate post operative period. CHA coatings are, therefore, the preferred coating and they are generally applied by plasma spraying or by sol-gel processing methods.
Such application methods are not, however, entirely satisfactory. With plasma spraying, which is a "line-of-sight" process it is extremely difficult, if not impossible, to apply a uniform coating to the irregularly shaped surface of a prosthetic device. Furthermore re-entrant and "backface" surfaces cannot be coated at all.
With sol-gel processing, it is somewhat easier to coat irregular surfaces but uniformity remains a problem and these is the problem that the coating must be sintered to remove the organic materials and densify the ceramic material. Local overheating of the metallic substrate may affect the physical properties--such as the fatigue and tensile strengths--of the substrate. Electrophoretic deposition of phosphate films onto titanium substrates also suffers from the fact that sintering of the film is required to provide a uniform adherent coating.
Thus, there is a need for an improved process for the deposition of adherent oxide and phosphate bioactive coatings onto conducting substrates of controlled thickness and porosity which do not require substrate heat treatment or sintering.